**1. Introduction**

90 Biogas

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Today, globally most energy is provided by burning oil and only a very small percentage is generated by nuclear power plants. The contribution of energy from renewable resources is almost negligible. But this will change in the future with increasing in environmental pollution and fossil fuel depletion, in addition to environmental problems generated by the Fukushima nuclear power plant.

One of the most attractive ways to obtain sources of alternative energy and the pollution control is the recover resource and energy from waste streams through bioconversion processes (Cantrell et al., 2008). In this respect, intensive studies have been conducted in the past few decades and various "green technologies" have been extensively reviewed (Kleerebezemand and Loosdrecht, 2007; Hallenbeck and Ghosh, 2009). For many years, anaerobic digestion has been a prevailing technology for biogas production, in which substrates are converted to methane and other products under a joint effort of several microbial groups in a reaction system (Sterling et al., 2001).

In this context biogas generated by agro-industrial wastewater will play a vital role in future. Biogas is a versatile renewable energy source, which can be used for replacement of fossil fuels in power and heat production, and it can be used also as gaseous vehicle fuel. Methane-rich biogas can replace also natural gas, as a feedstock in the production of chemicals and materials (Shin et al., 2010).

Sustainable development must be the foundation for economic growth in the twenty-first century. It is necessary redirect the efforts toward bioenergy production from renewable material, low-cost and locally available feedstock such as waste and wastewater agroindustrial. This effort will not only alleviate environmental pollution, but also reduce energy insecurity and demand for declining natural resources. The most cost-effective and sustainable approach is to employ a biotechnology option. Anaerobic treatment is a technology that generates renewable bioenergy necessary to replace the energy requirements around the world through the production of methane and hydrogen. However, it has also been employed for production of polyhydroxyalkanoates (PHA), these are linear polyesters generated by bacterial fermentation of sugar or lipids. They are produced by the bacteria to store carbon and energy. More than 150 different monomers can be combined within this family to give

Biogas Production from Anaerobic Treatment of Agro-Industrial Wastewater 93

One of the key factors in the success of microbial-mediated processes is an adequate understanding of process microbial, more specifically the study of microscopic organisms involved in wastewater degradation and byproduct formation. The low growth rate, the specific nutrient and trace mineral requirements of methanogens, coupled with their susceptibility to changes in environmental conditions demand meticulous process control for stable operation (Khanal, 2008). The biochemistry mainly involves enzyme-mediated chemical changes (the chemical activities of microorganism), type of substrate (kind wastewater) microorganism can destroy or transform to new compounds, and the step-by-

The anaerobic digestive process is a natural biological process in which an interlaced community of bacteria cooperates to obtain a stable and auto-regulated fermentation through assimilation, transformation and decomposition of the residual organic matter present in waste and wastewater into biogas. This is a complex multistep process in terms of chemistry and microbiology, where the organic material is degraded to basic constituents to obtain methane gas under the absence of an electron acceptor such as oxygen. The common metabolic pathway and process microbiology of anaerobic digestion is shown in Fig. 1

Generally, the anaerobic digestion process consists of four stages; the first one is called hydrolysis (or liquefaction), it consists in the transformation of complex organic matter such as proteins, carbohydrates and lipids into simple soluble products like sugars, long-chain fatty acids, amino acids and glycerin, this stage is carried out by the action of extracellular

In the second step, called the acidogenic stage fermentative bacteria use the hydrolysis products to form intermediate compounds like organic acids, including volatile fatty acids (VFA). Theses VFA along with ethanol are converted to acetic acid, hydrogen and carbon dioxide by other group of bacteria known as hydrogen-producing acetogenic bacteria

Organic acids are oxidized partially by bacteria called acetogenic in the third stage, which produce additional quantities of hydrogen and acetic acid. The acetogenesis is regarded as thermodynamically unfavorable unless the hydrogen partial pressure is kept below 10-3 atm, pathway efficient removal of hydrogen by the hydrogen-consuming organisms such as

Finally, in the fourth stage, both acetic acid and hydrogen are the raw material for the growth of methanogenic bacteria, converting acetic acid and hydrogen to biogas composed

Acetate, H2 and CO2 are the primary substrate for methanogenesis. On chemical oxygen demand (COD) basis about 72% of methane production comes from the decarboxylation of acetate, while the remainder is from CO2 reduction (McCarty, 1964). The groups of microorganisms involved in the generation of methane from acetate are known as acetotrophic or aceticlastic methanogens (group 3). The remaining methane is generated

**3. Fundamentals of microbiology and biochemistry in anaerobic digestion** 

step pathway of degradation (Sachdeva et al., 2000).

**3.1 Organics conversion in anaerobic systems** 

enzymes excreted by the fermentative (group 1) (Khanal, 2008).

hydrogenotrophic methanogens and/or homoacetogens (Zinder, 1988).

mainly of methane, carbon dioxide and hydrogen sulfide (Khanal, 2008).

(Khanal, 2008).

(group 2) (Khanal, 2008).

materials with extremely different properties. These plastics are biodegradeable and are used in the production of bioplastics (Mu et al., 2006) and other biochemicals.

This chapter intends to bring together the knowledge obtained from different applications of anaerobic technology in the treatment of various kinds of agro-industrial wastewaters to generate biogas. The first part covers essential information on the fundamentals of anaerobic technology, to demonstrate how the anaerobic treatment is able to generate significant volumes of methane-rich biogas. The wastewaters used in this chapter to generate biogas, contribute significantly in the pollution of the water bodies. In this opportunity the wastewater from Tequila vinasses were treated by different microbial consortia with energy purpose. This chapter illustrates the basics concepts of microbiology and biochemistry involved in the wastewater anaerobic treatment. The remainder focuses on various anaerobic reactor configurations and operating conditions used for the treatment of agroindustrial wastewaters different, show some examples with technical viability and the potential benefits that would be obtained by the utilization of the biogas as source of energy to full scale.
